The vast majority of thermoplastic polyurethane (TPU) fibers are made by a dry spinning process involving dissolving the TPU in a solvent. Melt spinning TPU fibers has been gaining in use in recent years. Melt spinning does not involve the use of a solvent, and therefore is more environmentally friendly.
Melt spinning TPU fibers involves feeding TPU polymer into an extruder and from the extruder to a spinneret where the fiber exits the spinneret. Polymers, such as TPU, tend to crystallize or crosslink so as to form lumps if they remain in melt processing equipment for too long. This is especially the case where crosslinking agents are added to the TPU prior to being fed to the spinneret. These early formed crystallized polymer and/or crosslinked polymer lumps may find their way through the spinneret and cause defects or undesirable properties in the fiber. Fiber breakage can also result. Lumps of crystallized and/or crosslinked material may also accumulate in the cavities upstream of the fiber opening. This can result in excessive back pressure and reduced material flow. Back pressure can build up to the point where fiber can no longer be produced. This requires shutting down the process of manufacturing the fiber and cleaning the equipment to remove the blocking material.
Another problem that can occur with melt spinning TPU fibers is that the modulus of the fibers can be too high for circular knitting applications.
Thus, there exists a need for improvements in melt spinning of elastomeric fibers.